# std::for_each

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt, class UnaryFunc > UnaryFunc for_each( InputIt first, InputIt last, UnaryFunc f ); | (1) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryFunc > void for_each( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryFunc f ); | (2) | (since C++17)

Applies the given function object f to the result of dereferencing every iterator in the range `[`first`,` last`)`. If f returns a result, the result is ignored.

1) f is applied in order starting from first.

If `UnaryFunc` is not MoveConstructible, the behavior is undefined. | (since C++11)

2) f might not be applied in order. The algorithm is executed according to policy.
This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

If `UnaryFunc` is not CopyConstructible, the behavior is undefined.

If the iterator type (`InputIt`/`ForwardIt`) is mutable, f may modify the elements of the range through the dereferenced iterator.

Unlike the rest of the parallel algorithms, `for_each` is not allowed to make copies of the elements in the sequence even if they are TriviallyCopyable.

### Parameters

first, last | - | the range to apply the function to
policy | - | the execution policy to use
f | - | function object, to be applied to the result of dereferencing every iterator in the range `[`first`,` last`)` The signature of the function should be equivalent to the following: void fun(const Type &a); The signature does not need to have const &. The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type. ​
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `ForwardIt` must meet the requirements of LegacyForwardIterator.

### Return value

1) f
2) (none)

### Complexity

Exactly std::distance(first, last) applications of f.

### Exceptions

The overload with a template parameter named `ExecutionPolicy` reports errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Possible implementation

See also the implementations in libstdc++, libc++ and MSVC stdlib.

    template<class InputIt, class UnaryFunc>
    constexpr UnaryFunc for_each(InputIt first, InputIt last, UnaryFunc f)
    {
        for (; first != last; ++first)
            f(*first);

        return f; // implicit move since C++11
    }

### Notes

For overload (1), f can be a stateful function object. The return value can be considered as the final state of the batch operation.

For overload (2), multiple copies of f may be created to perform parallel invocation. No value is returned because parallelization often does not permit efficient state accumulation.

### Example

The following example uses a lambda-expression to increment all of the elements of a vector and then uses an overloaded `operator()` in a function object (a.k.a., "functor") to compute their sum. Note that to compute the sum, it is recommended to use the dedicated algorithm std::accumulate.

Run this code

    #include <algorithm>
    #include <iostream>
    #include <vector>

    int main()
    {
        std::vector<int> v{3, -4, 2, -8, 15, 267};

        auto print = [](const int& n) { std::cout << n << ' '; };

        std::cout << "before:\t";
        std::for_each(v.cbegin(), v.cend(), print);
        std::cout << '\n';

        // increment elements in-place
        std::for_each(v.begin(), v.end(), [](int &n) { n++; });

        std::cout << "after:\t";
        std::for_each(v.cbegin(), v.cend(), print);
        std::cout << '\n';

        struct Sum
        {
            void operator()(int n) { sum += n; }
            int sum {0};
        };

        // invoke Sum::operator() for each element
        Sum s = std::for_each(v.cbegin(), v.cend(), Sum());
        std::cout << "sum:\t" << s.sum << '\n';
    }

Output:

    before:	3 -4 2 -8 15 267
    after:	4 -3 3 -7 16 268
    sum:	281

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 475 | C++98 | it was unclear whether f can modify the elements of the sequence being iterated over (`for_each` is classified as “non-modifying sequence operations”) | made clear (allowed if the iterator type is mutable)
LWG 2747 | C++11 | overload (1) returned std::move(f) | returns f (which implicitly moves)

### See also

transform | applies a function to a range of elements, storing results in a destination range (function template)
for_each_n (C++17) | applies a function object to the first N elements of a sequence (function template)
ranges::for_each (C++20) | applies a function to a range of elements (algorithm function object)
ranges::for_each_n (C++20) | applies a function object to the first N elements of a sequence (algorithm function object)
range-`for` loop(C++11) | executes loop over range

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